1. Field of the Invention
The present invention relates generally to a method and apparatus for cooling daughter boards containing electrical devices. More particularly, the present invention relates to a method and apparatus for conductively cooling electrical devices on a daughter board.
2. Description of the Related Art
In the field of high speed computers, high density, high speed electronic devices exhibit above average power consumption characteristics. That power consumption leads to correspondingly high levels of heat generation which must be effectively dissipated to avoid downtime for heat related failures. Any cooling apparatus or method used in conjunction with such high speed computers must be able to efficiently dissipate heat from the electronic devices while minimizing the effect of such heat dissipation on the electrical performance of the devices.
Commonly assigned U.S. Pat. No. 4,884,168 issued on Nov. 28, 1989 to August et al., titled "COOLING PLATE WITH INTERBOARD CONNECTOR APERTURES FOR CIRCUIT BOARD ASSEMBLIES" describes a cooling plate for heat dissipation that is particularly adapted for use with stacks of printed circuit boards. The cooling plate includes apertures and mounting means for Z-axis connector assemblies so that printed circuit boards attached to either side of the cooling plate may be electrically interconnected.
Externally, the cooling plate has a fixed pattern of heat conducted paths that are substantially identical to the pattern of devices on a printed circuit board attached to the cooling plate. As a result, the heat generated by the devices is thermally conducted directly to the cooling plate.
Commonly assigned U.S. Pat. No. 4,628,407 issued Dec. 9, 1986 to August et al., titled "CIRCUIT MODULE WITH ENHANCED HEAT TRANSFER AND DISTRIBUTION" describes a printed circuit board used in a high performance computer. A printed circuit board stack is disclosed which includes a heat conducting plate situated within the stack. The printed circuit boards contain thermally conductive paths from each circuit device mounted on the board and through the printed circuit board to locations in contact with the cooling plate. This system provides adequate heat dissipation properties, but tolerance variations in the thickness of the printed circuit board can affect the thickness of the thermal compound between the thermal connector and the circuit device. Those tolerance variations and resulting variations in thickness of the thermal compound can adversely affect the heat dissipation properties of this apparatus.
Commonly assigned U.S. Pat. No. 5,014,904 to Morton issued on May 14, 1991, and titled "BOARD MOUNTED THERMAL PATH CONNECTOR AND COLD PLATE" describes a method and apparatus to dissipate heat from printed circuit boards and electronic devices mounted on them. The printed circuit boards are provided with apertures which receive thermal conductor pads. The thermal conductor pads are secured in the apertures and integrated circuit devices are attached to one side of the pads while the other side of the pads are in contact with a cooling plate.
All of the above patents disclose cooling systems which are particularly well suited for use with printed circuit boards which lie on a substantially planar cooling plate or other heat sink. As such, conductive cooling paths from the devices producing heat to the cooling plates are easily established.
None of the devices, however, address the need for cooling electrical devices on daughter boards attached to such mother boards through the use of a thermally conductive heat transfer path. As a result, daughter boards having electrical devices are typically cooled using fluid mediums, such as liquid or gas, which convectively transfer heat away from the electrical devices. Convection is commonly described as a combination of fluid mixing and conduction. Forced-air convection (typically used for cooling) is not, however, as effective as pure conduction in the transfer of heat away from an electrical device.
In addition, daughter boards cooled through convection must be spaced apart to allow room for the cooling medium to flow around the daughter boards. As a result, the electrical devices are located at greater than optimum distances from the devices with which they communicate. That additional distance causes corresponding reductions in the speed of the computers as signals must travel further to the electrical devices on the mother boards.
Also, as the spacing between daughter boards increases, so must the distance between mother boards on adjacent cooling plates increase--which also causes reductions in computing speed of the machines.